underground tunnels what go up to trenches or an secret underground base like a rebel one in half-life 2

also you could add subways its up to you

That may be very hard to do but possible I'll try.

I have created a medium size underground base and a small "swamp" area. gm_bigconstruct 3 should be uploaded today.

I would suggest maybe a small-medium sized arena with some toggle switches to spawn waves of enemies. For instance, one that will place the different kinds of zombies, one for the Combine, one for the Antlions, and finally one for rebels. So you can have a fight with any combination of the four factions. It could be fun, but if it's too much trouble then just an arena where we can manually spawn things.

can you please fix the roofs of the houses and the walls of the map and the windows of the houses cause they are all giving me a neon purple colour ;_

Purple neons = type mat_specular 0 in console.

Arena you say? Well I can think of making it. Great idea

ANNOUNCEMENT: gm_bigconstruct 3 is almost ready. I just have to fix dynamic soundscape and elevator to second underground base. It added 2 underground bases. Both are cool but the second one better :D hope you like it guys!

I look forward to it.

gm_bigconstruct 3 is uploaded now! Feel free to check it out!